Rename z_lo, z_up to z_pre_relu_lo, z_pre_relu_up

[hongkai-dai]

In relu_to_optimization.py, ReLUFreePattern.output_constraint() returns z_lo, z_up, this name is confusing, as it is NOT the bounds on z (which is after the ReLU activation). This is the bound on the signal before the ReLU activation.